1. Field of the Invention
This invention relates to an optical recording medium adapted to record information in a recording layer by irradiating a light beam thereto to induce a crystal-lographic change.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable optical recording medium is of the phase change type wherein laser light is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by a single light beam and operated by a drive unit with a relatively simple optical system as compared with magneto optical recording media.
Most optical recording media of the phase change type used Ge-Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as defined in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe.sub.2 phase.
Japanese Pat. application Nos. 108996/1992, 79267/1992, 253832/1992, 17968/1993, 341818/1993, and 87854/1994 by the inventors disclose phase change optical recording media having recording layers of a quaternary system of Ag, Sb, Te, and In to which V, Ti or the like is added for imparting stability to the recording layer at a relatively low linear velocity range of about 1.2 to 2.8 m/s for improving reliability.
In the case of high density image recording, however, it is necessary to increase the linear velocity for accomplishing high speed overwriting. Overwriting of a recording layer composed mainly of Ag, Sb, Te, and In is carried out by applying a recording power to spots where record marks are to be formed while adding a biasing power to the laser light source. The biasing power is weaker than the recording power. Application of the biasing power causes amorphous portions to crystallize and crystalline portions to re-crystallize whereby previous record marks are erased to resume the initialized state. Since the cooling rate after irradiation of laser light depends on the linear velocity of the medium, the recording layer must have a crystal transition rate (the rate at which an amorphous or microcrystalline material grows into coarse crystals) corresponding to the linear velocity before such overwriting can be done. To achieve high speed overwriting by increasing the linear velocity, the crystal transition rate of the recording layer must be accelerated. This may be achieved by increasing the relative content of antimony (Sb) and decreasing the relative content of tellurium (Te). Even when the adjustment of Sb and Te is employed, high speed overwriting encounters a certain limit above which erasing properties are unsatisfactory. Additionally, a recording layer with increased Sb and decreased Te has a lower activation energy so that record marks are likely to crystallize, indicating that the medium is less reliable during storage at elevated temperature. For example, those media having vanadium added to the recording layer for reliability improvement which are recorded at a linear velocity of 4 m/s or higher have the problem that amorphous record marks crystallize in about 200 hours during storage in an environment at 80.degree. C.